The Problem of Iodine-125 for Treatment of Thyrotoxicosis

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Abstract
The fortuitous results of experiments showing a higher incidence of tumors after the administration of 125I than after 131I, raised the question of the relative biological effectiveness of 125I. A reevaluation of the microdosimetry of 125I in the thyroid follicle revealed that due to the low energy electrons–especially the Auger electrons–the radiation dose delivered to the cell colloid interface is ca. 4 or more times greater than that from an equivalent amount of 131I. At the same time, the nucleus dose of the cell is reduced by up to 80% as compared to that of 1311. 125I was therefore suggested as a suitable substitute for the treatment of thyrotoxicosis. On the basis of the radiation dose delivered to the nucleus, a dose of 40 mCi on the average was proposed. On the other hand, basing the dose on the radiation delivered to the colloid-cell boundary, which is probably the site of the sensitive hormonogenic mechanism, much smaller amounts, in the range of 2–3.5 mCi 125I were administered. The results obtained in this series of 35 patients, with a follow-up of up to a year have so far partially justified this approach. A 20% failure was noted which suggests the use of carefully adjusted higher doses, most probably in the range of 4–6 mCi.

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